Substrate of gallium nitride single crystal and process for producing the same

ABSTRACT

The present invention relates to a method for producing an epitaxial substrate having a III-V group compound semiconductor crystal represented by the general formula In x Ga y Al z N (wherein, x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1) having reduced dislocation density, comprising a first step of covering with a mask made of a different material from the III-V group compound semiconductor so that only portions around points of the crystal constitute openings by using a III-V group compound semiconductor crystal having a plurality of projection shapes and a second step of growing the III-V group compound semiconductor crystal laterally by using the III-V group compound semiconductor crystal at the opening as a seed crystal. According to the present invention, an epitaxial substrate having a III-V group compound semiconductor crystal having low dislocation density and little warp is obtained.

FIELD OF THE INVENTION

The present invention relates to a gallium nitride single crystalsubstrate and method for producing the same.

BACKGROUND ART

Nitride-based III-V group compound semiconductors represented by thegeneral formula In_(x)Ga_(y)Al_(z)N (wherein, x+y+z=1, 0≦x≦1, 0≦y≦1,0≦z≦1) have controllable direct type band gap corresponding to fromultraviolet to red depending on the composition of a III group element,consequently, can be utilized as a material for a light emitting deviceof high efficiency ranging from ultraviolet to visible light. It istheoretically possible to manufacture an electronic device excellent inenvironment resistance utilizing a property as a semiconductor even athigh temperatures under which conventional semiconductors cannotoperate, due to larger band gap as compared with semiconductors such asGaAs and the like generally used up to now.

However, nitride-based III-V group compound semiconductors cannot easilyperform crystal growth of a bulk single crystal, and free standingsubstrates of practically endurable nitride-based III-V group compoundsemiconductors are still under developing. Therefore, substrates widelyused currently are sapphire and the like. Usually, methods of epitaxialgrowth such as a metal-organic chemical vapor deposition method(hereinafter, abbreviated as MOCVD method) are used.

However, a sapphire substrate has a lattice constant differentsignificantly from that of a nitride-based III-V group compoundsemiconductor, resultantly, it is impossible to grow directly on this acrystal of a nitride-based III-V group compound semiconductor.Therefore, there is invented and usually used a method of growingamorphous GaN, AlN or the like once at lower temperature, relaxinglattice strain, and then, growing a crystal of a nitride-based III-Vgroup compound semiconductor on this (Japanese Patent ApplicationLaid-Open (JP-A) No. 63-188983). By this method, the quality of acrystal of a nitride-based III-V group compound semiconductor hasincreased dynamically.

However, since a discrepancy of lattice constant between a sapphiresubstrate and a crystal of a nitride-based III-V group compoundsemiconductor is not resolved, dislocation, this is a crystal defect, isstill present at a density of as high as 10⁹ to 10¹⁰ cm⁻² in the crystalof a nitride-based III-V group compound semiconductor. This dislocationis a problem since it remarkably decreases the performance of an devicesuch as life or the like.

Then, recently, as a method of reducing dislocation generating based onthe discrepancy of lattice constant from sapphire, there is suggested amethod in which on GaN having dislocation present in high density, amask patterned with SiO₂ and the like is formed, GaN is grown from awindow portion of the mask and the mask is covered by lateral growth toobtain a flat GaN crystal, and there is also reported that dislocationdensity can be decreased to 10⁷ cm⁻² by blocking dislocation from thetemplate using a mask (Appl. Phys. Lett. 71(18) 2637 (1997)).

On the other hand, as a method of obtaining a free standing GaNsubstrate, a method is reported in which on a sapphire substrate and thelike, a GaN crystal is epitaxially grown, and sapphire and the like areremoved by using etching or laser (Jpn. J. Appl. Phys. vol. 38, p.L217-219 (1999), JP-A No. 2000-129000).

However, this method has a problem that because of a difference inthermal expansion coefficient between sapphire and the like and GaN,warp occurs in a cooling step after growth, consequently, warp or crackremains on the resulted free standing substrate, further, a problem thatdislocation density sufficiently reduced cannot be obtained.

As a method of solving these problems, there is suggested a method inwhich a metal thin film made of Ti and the like having a catalyticaction promoting decomposition of GaN is formed on the surface of GaN,then, a thermal treatment is conducted on this under an atmospherecontaining NH₃, to form TiN in the form of mesh on GaN andsimultaneously to form a void of reverse cone shape on the ground GaN inthe mesh space, and GaN is laterally grown on this TiN, then, this ispeeled by using mixed liquid of hydrofluoric acid and nitric acid toobtain a free standing substrate having a dislocation density reduced toabout 10⁷ cm⁻² and little warp. (JP-A No. 2002-343728)

However, also this method has a problem that warp is not sufficientthough it is decreased and nitriding of Ti and formation of a void inGaN are conducted simultaneously in a thermal treatment, consequently,control of the void proportion and adjustment of the degree of nitridingof Ti and the like are difficult and stable production of a substrate oflow dislocation is difficult, a problem that mixed liquid ofhydrofluoric acid and nitric acid is necessary for conducting peeling,and other problems.

DISCLOSURE OF THE INVENTION

The present inventors have intensively studied for solving theabove-mentioned problems and resultantly found that a nitride-basedIII-V group compound semiconductor crystal having a specific structurecan be formed in which a point in the form of projection is used as aseed crystal, the whole surface of the crystal is flat, and voids areleft internally regularly and periodically, by using a nitride-basedIII-V group compound semiconductor crystal having a plurality ofprojection shapes, masking portions other than the points in the form ofprojection with a mask, then, growing the crystal laterally, that thiscrystal has dislocation density equal to or less than the level obtainedin conventional methods, further that, by controlling thecross-sectional area of the point in the form of projection to a certainlevel or less, a template and an upper crystal can be peeled easilywithout using etching or laser after crystal growth and a free standingsubstrate having low dislocation density and showing extremely smallwarp can be obtained, leading to completion of the invention.

Namely, the present invention provides

[1] A method for producing an epitaxial substrate having a III-V groupcompound semiconductor crystal represented by the general formulaIn_(x)Ga_(y)Al_(z)N (wherein, x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1) havingreduced dislocation density, comprising a first step of covering with amask made of a different material from the III-V group compoundsemiconductor so that only portions a round points of the crystalconstitute openings by using a III-V group compound semiconductorcrystal having a plurality of projection shapes and a second step ofgrowing the III-V group compound semiconductor crystal laterally byusing the III-V group compound semiconductor crystal at the opening as aseed crystal,

[2] A method for producing a free standing substrate of a III-V groupcompound semiconductor crystal represented by the general formulaIn_(x)Ga_(y)Al_(z)N (wherein, x+y+z=1, 0≦x>1, 0≦y≦1, 0≦z≦1) havingreduced dislocation density, comprising a first step of covering with amask made of a different material from the III-V group compoundsemiconductor so that only portions around points of the crystalconstitute openings and the sum of the areas of the openings is ½ orless of the area projected from the upper direction of the III-V groupcompound semiconductor crystal having a plurality of projection shapesby using a III-V group compound semiconductor crystal having a pluralityof projection shapes and a second step of growing the III-V groupcompound semiconductor crystal laterally by using the III-V groupcompound semiconductor crystal at the opening as a seed crystal,

[3] The method according to [1] or [2], wherein the III-V group compoundsemiconductor crystal having a plurality of projection shapes in thefirst step is obtained by using the III-V group compound semiconductorcrystal as a template, covering this with a mask having a plurality ofopenings, then, selectively growing the III-V group compoundsemiconductor crystal from the opening so as to form an oblique facetagainst the surface of the template,

[4] A III-V group compound semiconductor epitaxial substrate containinga void surrounded by a plane parallel to a substrate and an obliqueplane covered with a different material from a III-V group compoundsemiconductor, and

[5] A free standing substrate of a III-V group compound semiconductorcrystal having reduced dislocation density obtained by the methodaccording to [2].

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a schematic view showing a process for producing a GaN freestanding substrate showing one example of the present invention.

FIG. 2 (A) is a view showing one example of a device utilizing anepitaxial substrate of the present invention, (B) is a viewschematically showing the condition of reduction in dislocation densityin the present invention, and (C) is a view schematically showing amethod of reducing dislocation density by repetition of lateral growth.

FIG. 3 (A) shows a mask pattern used in covering a template in Example3, and (B) shows a mask pattern used in covering a template in Example4.

FIG. 4 is a schematic view showing a process for producing a freestanding substrate of the present invention.

FIG. 5 is a schematic view showing a process for producing a freestanding substrate of the present invention.

DESCRIPTION OF MARKS

-   -   1: ground GaN epitaxial crystal    -   2: pattern    -   2A: mask portion    -   2B: opening    -   3: crystal having projection shape    -   4: mask used in first step    -   5: photoresist layer    -   6: GaN opening (exposed portion, seed crystal)    -   6′: connection portion    -   7: grown crystal    -   7A: association portion    -   7′: free standing substrate    -   8: epitaxial crystal having void internally    -   8A: void    -   9: n type layer    -   10: light emitting layer    -   11: p type layer    -   12: n electrode    -   13: p electrode

BEST MODES FOR CARRYING OUT THE INVENTION

As the method of growing a III-V group compound semiconductor crystalrepresented by the general formula In_(x)Ga_(y)Al_(z)N (wherein,x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1) in the present invention, a HVPE methodand a MOVPE method are suitably used. The HVPE method can obtain anexcellent crystal in a short period because of large growth speedobtainable and resultantly can be used suitably in the presentinvention. The MOVPE method can effect crystal growth with gooduniformity on a lot of substrates and resultantly can also be usedsuitably in the present invention. These methods can be conducted incombination, and for example, the step of growing a III-V group compoundsemiconductor crystal having a plurality of projection shapes may beconducted by the MOVPE method and the second step, namely, lateralgrowth for obtaining a III-V group compound semiconductor crystal havingreduced dislocation density may be conducted by the HVPE method, in thepresent invention.

As conditions for effecting crystal growth, temperature, pressure,carrier gas, raw material and the like are important, and conventionallyknown conditions of them can be used.

For example, in the second step, the growth pressure is usually 0.001atm or more. In this step, crystallinity tends to lower when thepressure is 0.001 atm or less. It is preferably 0.005 atm or more,further preferably 0.01 atm or more. Crystallinity is improved in somecases when the growth pressure is higher, and a MOVPE apparatus or HVPEapparatus generally used for crystal growth is not used industrially atso high growth pressure, therefore, the growth pressure in re-growth ispreferably 10 atm or less.

Regarding the carrier gas, those used in usual MOVPE and HVPE such ashydrogen, nitrogen, helium, argon and the like can be used. As the rawmaterial, conventionally known materials can be used.

The process of the present invention will be illustrated below usingFIG. 1.

The present invention is characterized in that a III-V group compoundsemiconductor crystal having a plurality of projection shapes is used,and the projection shape means a convex shape formed as oblique facetand having no plane parallel to the ground substrate, and includes thathaving a point extending linearly and that having a point in the form ofdot.

Such a III-V group compound semiconductor crystal having a plurality ofprojection shapes can be produced, for example, as shown in FIGS. 1 (A)and (B).

That is, as the ground substrate 1, a GaN epitaxial crystal carrying apattern formed is used suitably. Here, the substrate is not limited to aGaN epitaxial crystal, and a GaN buffer layer or AlN buffer layer grownat lower temperature carrying a pattern formed may be used to obtain theeffect of the present invention in some cases.

As the material used in pattern formation, in usual, materials differentfrom the III-V group compound semiconductor crystal are suitably used.These materials are required to stand a high temperature atmospherecontaining ammonia used for growth of a crystal in the form ofprojection, and examples thereof include metal oxide made of SiO₂, TiO₂and the like, nitride made of Si₃N₄, BN (boron nitride) and the like,single metal made of W (tungsten), Mo (molybdenum), Cr (chromium), Co(cobalt), Si (silicon), gold, Zr (zirconium), Ta (tantalum), Ti(titanium), Nb (niobium), nickel, platinum, V (vanadium), Hf (hafnium),Pd (palladium) and the like, and laminated structures of them.

In addition, that having an irregular pattern formed on the surface of aGaN epitaxial crystal by dry etching and the like, and that having anirregular pattern formed on a substrate made of sapphire and the like bydry etching and the like can also be used to obtain the effect of thepresent invention in some cases.

As the pattern form, those conventionally known can be used.Specifically, that having stripe-shaped masks having a constant widtharranged in parallel via an opening of constant width generally calledline/space, alternatively, those having a ground partially exposed inthe form of circle or polygon, and the like are mentioned. These patternforms can be selected depending on the subsequent growth conditions andpattern form.

In the case of a line/space pattern, the width of a mask portion ispreferably 0.05 μm or more and 20 μm or less. When the width of a maskportion is smaller than 0.05 μm, the effect of reduction in defectdensity of the present invention is not remarkable. When larger than 20μm, time required for embedding of a mask portion becomes too long,impractically. Due to the same reason, the distance between openings ispreferably 0.05 μm or more and 20 μm or less also in the case of apattern having an opening in the form of circle or polygon.

In the case of a line/space pattern, the width of an opening (exposedportion of ground) is preferably 0.01 μm or more and 20 μm or less. Whenthe width of an opening is smaller than 0.01 μm, current semiconductorprocesses are not preferable since it is not easily fabricated inpractically correct form. When larger than 20 μm, the effect ofreduction in defect of the present invention is not remarkable. Due tothe same reason, the size of an opening is preferably 0.01 μm or moreand 20 μm or less also in the case of a pattern having an opening in theform of circle or polygon. FIG. 1 shows schematically a line/spacepattern in which the width of a mask portion 2A and the width of anopening 2B are approximately the same.

In the case of a line/space pattern, preferable stripe direction is a<1-100> direction or <11-20> direction of a hexagonal GaN crystal.Particularly preferable is a <1-100> direction.

Here, the pattern is not limited to a stripe along one direction, and apattern superimposed stripes along a plurality of directions may also beused. For example, by using stripes along two directions, that havingprojection structures having a point in the form of dot arrangedtwo-dimensionally in disaggregation mode, and that having projectionstructures having a point in the form of line crossed along twodirections, and the like can be produced.

Also in the case of use of stripes along a plurality of directions, thestripe direction is preferably a <1-100> direction or <11-20> directionof a hexagonal GaN crystal. Because of symmetry of a nitride crystal in(0001) C plane, these directions constitute crystallographicallyequivalent direction every 60°. That is, two <1-100> directions crossingat 60°, two <11-20> directions crossing at 600, a combination of a<1-100> direction and a <11-20> direction crossing at 90°, and the likecan be suitably used.

For forming projection shapes, growth conditions for easy formation ofoblique facet are usually used. Specifically, the above-mentioned shapecan be formed easily when the growth temperature is relatively lower,for example, when it is 1050° C. or less, when the ratio of the feedingamount of a V group raw material to the feeding amount of a III groupraw material is larger, when an opening of a pattern is smaller ascompared with the width of a pattern, and when the growth pressure ishigher. By conducting re-growth until disappearance of a plane parallelto the surface of a template in a crystal grown under such conditions, acrystal 3 having projection shapes formed only of oblique facet isobtained. Even after disappearance of a plane parallel to the surface ofa template in a long crystal, growth may also be continued only withoblique facet by continuing growth under the same condition.

The first step of the present invention is a step of using a III-V groupcompound semiconductor crystal having a plurality of projection shapesas described above and covering with a mask made of a different materialfrom the III-V group compound semiconductor so that only portions aroundpoints of the crystal constitute openings, and the mask formation methodincludes (1) a method of forming a mask so as to cover the whole surfaceof projection shapes, then, removing the mask only at a point, (2) amethod of forming a pattern with a photoresist and the like so that onlya point is covered with a mask material, then, forming a mask material,and removing a mask material at portions other than the point(so-called, lift off method), and other methods. For samples havingprojection shapes, the method (1) is suitably used.

Namely, because of projection shapes, the thickness of an applied layer5 of a photoresist is smaller around a projection point and largeraround a lower part, therefore, when oxygen plasma ashing is conducted,a photoresist layer disappears preferentially from a portion of smallerphotoresist thickness (namely, projection point) to expose a mask 4.Thereafter, by etching the mask, a GaN semiconductor can be exposed onlyaround a projection point (FIGS. 1 (C), (D)). The area of an opening(exposed portion) 6 at a projection point can be controlled by adjustinglayer thickness distribution determined depending on applicationconditions of a photoresist, oxygen plasma ashing time and the like.Here, as the material used in a mask 4, the same materials as shown inproduction of the above-mentioned crystal having projection shapes canbe used.

The second step of the present invention is a step of using a III-Vgroup compound semiconductor crystal at the opening obtained asdescribed above as a seed crystal and growing the III-V group compoundsemiconductor crystal laterally, and for obtaining a flat layer 7,growth may be advantageously conducted under conditions wherein lateralgrowth is dominant than growth along the longitudinal direction. Undersuch conditions, a (0001) plane is easily formed, and theabove-mentioned conditions for growth a crystal having projection shapesare preferably conditions in which main condition factors are shifted tothe opposite side. Specifically, a flat layer is obtained easily whenthe growth temperature is relatively higher, for example when it is 900°C. or more, when the ratio of the feeding amount of a V group rawmaterial to the feeding amount of a III group raw material is smaller,when the growth pressure is relatively lower, for example when it is 2atom or less.

As the crystal growth method of the second step, HVPE giving largegrowth speed is suitably used, and additionally, a method combining twogrowth methods may also be used in which MOVPE manifesting good formcontrollability is used until a flat plane is obtained, then, thathaving larger thickness is produced using HVPE.

A crystal 7 having flat whole surface produced here is connected with atemplate only at a connection portion 6′ which is originally a seedcrystal portion, therefore, lattice strain ascribable to a difference inthermal expansion coefficient from a ground substrate made of sapphireand the like concentrates at this portion, and resultantly, cracking isformed at this portion and peeling from a template naturally occurs insome cases. Conditions enabling peeling without such special treatmentdepend on the thickness of a ground substrate made of sapphire and thelike, the thickness of a crystal grown in the second step, the areaproportion of an opening (area proportion of a seed crystal portion),namely, the proportion of the sum of the areas of mask openings to thearea projected from the upper direction of the III-V group compoundsemiconductor crystal having a plurality of projection shapes, and thelike. Qualitatively, natural peeling easily occurs when the crystalthickness in the second step is larger and the area proportion of anopening is smaller.

In the case of a sapphire ground substrate usually used having athickness of about 400 μm, the thickness of a crystal grown in thesecond step is preferably 10 μm or more, and more preferably 20 μm ormore. The area proportion of an opening is preferably ⅔ or less, morepreferably ½ or less. When smaller than 10 μm, when the area proportionis larger than ⅔, and the like, there is a tendency that natural peelingis not caused easily.

A free standing substrate thus naturally peeled is not influencedsignificantly by a ground substrate, consequently, warp thereof isextremely small. In the case of no natural peeling, it is also possibleto cause peeling by applying mechanical stress or thermal stress toobtain a free standing substrate.

The step of covering with a mask so that only portions around points ofa crystal having projection shapes constitute an opening as the firststep can use steps, for example, as shown in FIGS. 4 and 5 in additionto the above-mentioned step. The example of FIG. 4 is a method in whicha ground substrate carrying a pattern formed is dug by a method such asdry etching and the like to form projection shapes, then, a second maskis formed so that only portions around the points of the projectionshapes constitute an opening. In this method, growth of a III-V groupcompound semiconductor may be conducted only once.

The example of FIG. 5 is a method in which the thickness of a mask to beformed on a ground substrate is made relatively larger, a concave isformed on this mask, then, a flat crystal having a void internally isobtained by selective growth and lateral growth. In this method, growthis conducted under conditions wherein oblique facet is formed inselective growth, then, the growth conditions are changed so thatlateral growth is dominant before growing.

Thus, a free standing GaN substrate 7′ can be obtained. When freestanding GaN substrate is used, by conducting homo-epitaxial growth ofGaN on this by a HVPE method and the like, the thickness of a GaNsubstrate of low dislocation can be sufficiently increased and use as aningot can be made possible.

Further, by increasing the area proportion of a seed crystal (opening 6)in the first step of the present invention, a crystal 8 having aspecific structure containing avoid 8A left internally can be obtainedwithout peeling of a GaN layer. There is a possibility of producing anovel element as shown in FIG. 2 (A) by utilizing this structure. FIG. 2(A) shows a light emitting diode containing a metal layer of high lightreflectance as the mask 4 used in the first step, embedded in a crystal.By this, a light exiting toward lower direction from a light emittinglayer 10 can be reflected toward upper direction to obtain LEDmanifesting improved light emergence efficiency.

A GaN opening 6 around a crystal point formed in the first step acts asa seed crystal for crystal growth in the second step for forming a flatlayer. A crystal grown laterally from a seed crystal takes over thedirection of the seed crystal, resultantly, there are littledislocations in this. Since the most of dislocations taken over from atemplate are refracted along the horizontal direction internally andended at an oblique facet plane, in a step of growing a projection shape3, the dislocation of a template taken over to a flat layer 7 re-grownexists substantially only at a projection point as a seed crystal.Consequently, the dislocation density of the whole flat layer can bereduced effectively.

Dislocation transmitted from a template to a flat layer occurs only at aseed crystal portion as described above, however, dislocation occursnewly at an associated portion 7A of the flat layer in some cases. Thereason for this is that due to fluctuation of a crystal axis in a (0001)plane of a seed crystal, the orientations of mutually adjacent seedcrystals shift slightly and a grain boundary of small inclined angle isgenerated at an association portion of a laterally grown portion, and adislocation is arranged along this. This condition is shownschematically in FIG. 2 (B).

By repeating embedded growth by lateral growth, new dislocationsoccurring at an association portion can be reduced in some cases. Forreducing dislocations on a grain boundary of small incline angleoccurring at an association portion, it is necessary to increasedistance until association by lateral growth and to increase the size ofindividual grains surrounded by the grain boundary of small inclineangle. For conducting this, it may be advantageous that in a patternused before the first step conducted at second time, the period of thepattern is made larger than the period of the first time. Thus, theremaining dislocation density can be further reduced. This condition isschematically shown in FIG. 2 (C). As the method of embedded lateralgrowth, the second or later steps in the present invention may be usedas they are, however, more simply, it may be also permissible that apattern is formed on a flat layer obtained in the first time andembedded lateral growth is only conducted directly on this.

The present invention will be illustrated more specifically by examplesbelow, however, the scope of the invention is not limited only to theseexamples.

EXAMPLE 1

A SiO₂ mask formed by a vapor deposition method using a sample obtainedby forming an un-doped GaN layer 1 with a thickness of about 3 μm on aGaN buffer layer grown at lower temperature by a MOCVD method on asapphire (0001) plane substrate having a thickness of 430 μm was workedby a usual photolithography method so as to form stripe patterns alongthe <1-100> direction of the GaN crystal. The width of a SiO₂ stripeportion 2A and the width of a window portion 2B are both 5 μm.

This was placed on a MOCVD reaction furnace and crystal growth in thefirst step was conducted using H₂ as a carrier gas and using TMG and NH₃at a growth pressure of 0.66 atm and a growth temperature of 950° C., togrow GaN₃ having projection shapes composed of {11-22} plane facet.

Next, the first step was conducted to produce a sample with a SiO₂ maskhaving openings only at portions around projection portions. That is,first, the whole surface was covered with a SiO₂ layer 4 having athickness of 100 nm by a RF sputtering method. Next, a photoresist layer5 was formed by spin coating and baking, the photoresist at theprojection points was removed by an oxygen plasma ashing apparatus toexpose a SiO₂ layer at this portion. Then, the exposed SiO₂ portion wasremoved by conducting a buffered hydrofluoric acid treatment. Finally,the photoresist was removed with an organic solvent. Thus, a sample witha SiO₂ mask having openings only at projection point portions 6 obtainedin the first step was made. The area proportion of the openings wasabout 40%.

Next, the sample obtained in the first step was placed on a MOCVDapparatus and crystal growth in the second step was conducted to obtaina flat GaN crystal having a thickness of 3 μm. Conditions in thiscrystal growth include a growth pressure of 0.66 atm and a growthtemperature of 1050° C.

Based on a sectional SEM photograph of the resulted crystal, formationof a crystal was confirmed in which voids having oblique facet at theside surface covered with SiO₂ were left internally. As a result ofevaluation by cathode luminescence, dislocations were concentrated at anassociation portion of lateral growth and the dislocation density was1×10⁵ cm⁻² or less at portions other than the association portion. Theaverage dislocation density of the whole crystal was 1×10⁷ cm^(−2.)

EXAMPLE 2

Crystal growth was conducted in the same manner as in Example 1 exceptthat a HVPE method was used instead of a MOCVD method and the thicknessof the grown crystal was 100 μm in the crystal growth method in thesecond step. The growth conditions for HVPE included use of N₂ as acarrier gas and use of ammonia, hydrogen chloride gas and gallium metalas raw material, and a growth pressure of normal pressure and asubstrate temperature 1070° C.

When the sample completed growth was removed from there action furnace,the layer grown in the third step was peeled naturally to obtain a freestanding substrate.

As a result of evaluation of the resulted crystal by cathodeluminescence, dislocations were concentrated at an association portionof lateral growth and the dislocation density was 1×10⁵ cm⁻² or less atportions other than the association portion. The average dislocationdensity of the whole crystal was 5×10⁷ cm⁻².

As a result of evaluation of warp, the curvature radius of warp wasabout 2 m, confirming an extremely flat free standing substrate.

EXAMPLE 3

As the mask patterns for growth of a crystal having projection shapes, apattern having superimposed two stripes along the crystallographicallyequivalent <1-100> direction crossing mutually at an angle of 60° wasused instead of a stripe pattern along one direction. In one direction,the width of the mask portion and the width of the opening are both 5μm, and in another direction, the width of the mask portion and thewidth of the opening are 7 μm and 3 μm, respectively. As shown in FIG. 3(A), it is a mask in which mask portions of parallelogram shape arearranged regularly in disaggregation mode in openings.

Crystal growth was conducted in the same manner as in Example 2 exceptthat the oxygen plasma ashing time was controlled so that the areaproportion of openings of the mask in the first step was 70%, inaddition to the above-mentioned variations.

As a result of evaluation of the resulted crystal by cathodeluminescence, dislocations were concentrated at the center of the maskand different facet-associated portions (that is, association portion oflateral growth), and the dislocation density was 1×10⁵ cm⁻² or less atportions other than the association portion. The average dislocationdensity of the whole crystal was 3×10⁷ cm⁻².

EXAMPLE 4

As the mask patterns for growth of a crystal having projection shapes, apattern of a stripe along a <1-100> direction and two stripes mutuallycrossing along a <11-20> direction superimposed was used. The width ofthe mask portion of a stripe along a <1-100> direction and the width ofthe opening are both 5 μm, and the width of the mask portion along a<11-20> direction and the width of the opening are 7 μm and 3 μm,respectively. As shown in FIG. 3 (B), it is a mask in which maskportions of rectangle shape are arranged regularly in disaggregationmode in openings.

Crystal growth was conducted in the same manner as in Example 3 exceptthat the oxygen plasma ashing time was controlled so that the areaproportion of openings of the mask in the first step was 70%, inaddition to the above-mentioned variations.

As a result of evaluation of the resulted crystal by cathodeluminescence, dislocations were concentrated at the center of the maskand different facet-associated portions (that is, association portion oflateral growth), and the dislocation density was 1×10⁵ cm⁻² or less atportions other than the association portion. The average dislocationdensity of the whole crystal was 7×10⁷ cm^(−2.)

According to the present invention, a free standing substrate of anitride-based III-V group compound semiconductor single crystal havingsmall dislocation density and manifesting small warp can be obtained.This free standing substrate of a nitride-based III-V group compoundsemiconductor single crystal can be used widely as a substrate for anitride-based III-V group compound semiconductor device, and enablesproduction particularly of ultraviolet ray emitting LED and laser diodehaving high reliability, that is, this free standing substrate isextremely useful.

1. A method for producing an epitaxial substrate having a III-V groupcompound semiconductor crystal represented by the general formulaIn_(x)Ga_(y)Al_(z)N (wherein, x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1) havingreduced dislocation density, comprising a first step of covering with amask made of a different material from the III-V group compoundsemiconductor so that only portions around points of the crystalconstitute openings by using a III-V group compound semiconductorcrystal having a plurality of projection shapes and a second step ofgrowing the III-V group compound semiconductor crystal laterally byusing the III-V group compound semiconductor crystal at the opening as aseed crystal.
 2. A method for producing a free standing substrate of aIII-V group compound semiconductor crystal represented by the generalformula In_(x)Ga_(y)Al_(z)N (wherein, x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1)having reduced dislocation density, comprising a first step of coveringwith a mask made of a different material from the III-V group compoundsemiconductor so that only portions around points of the crystalconstitute openings and the sum of the areas of the openings is ½ orless of the area projected from the upper direction of the III-V groupcompound semiconductor crystal having a plurality of projection shapesby using a III-V group compound semiconductor crystal having a pluralityof projection shapes and a second step of growing laterally the III-Vgroup compound semiconductor crystal by using the III-V group compoundsemiconductor crystal at the opening as a seed crystal.
 3. The methodaccording to claim 1 or 2, wherein the III-V group compoundsemiconductor crystal having a plurality of projection shapes in thefirst step is obtained by using the III-V group compound semiconductorcrystal as a template, covering this with a mask having a plurality ofopenings, then, selectively growing the III-V group compoundsemiconductor crystal from the opening so as to form an oblique facetagainst the surface of the template.
 4. A III-V group compoundsemiconductor epitaxial substrate containing a void surrounded by aplane parallel to a substrate and an oblique plane covered with adifferent material from a III-V group compound semiconductor.
 5. A freestanding substrate of a III-V group compound semiconductor crystalhaving reduced dislocation density obtained by the method according toclaim 2.